Assemblies of carbon, derived from a variety of carbon sources, have a multitude of current and anticipated commercial, industrial, and high-technology applications. For example, activated charcoal or activated carbon, which is usually in the form of loose powder, particles, or irregular agglomerates, has a variety of uses in filtration and catalyst support. This material has also recently been explored for energy storage applications, as an ionic exchange medium or supercapacitor electrode material. Graphite in its various forms has numerous uses, for example, as refractory material, in brake linings, and as electrodes in electric arc furnaces. Intercalated graphite and expanded graphite have been studied for use as fire retardants and high temperature applications. These carbon assemblies have many desirable properties such as resistance to chemical attack, resistance to high temperatures, and high surface area in the case of activated carbon, and electrical conductivity and lubricity in the case of graphite. However, these materials typically require a binder or matrix material to form them into a cohesive assembly of a desired shape and size, having good mechanical strength and integrity.
More recently, assemblies of carbon nanotubes (CNTs) in various forms have attracted much attention and are being explored and developed for diverse applications. Such assemblies have been referred to in the literature as “buckypaper” or “buckydiscs”. For example, Dharap et al in “Nanotube film based on single-wall carbon nanotubes for strain sensing”, Nanotechnology 15 (2004), pp. 379-382, investigate the use of isotropic films of randomly oriented CNTs as mechanical strain sensors. Cao et al, in “Random networks and aligned arrays of single-walled carbon nanotubes for electronic device applications,” Nano Research 1, 4 (2008), pp. 259-272, discuss the use of random networks or aligned arrays of CNTs as thin-film transistors. Ma et al, in “Methods of making carbide and oxycarbide containing catalysts,” U.S. Pat. No. 7,576,027 B2, disclose catalyst supports for fluid phase chemical reactions made from randomly entangled CNT aggregates. And Liu et al, in “Electrochemical capacitor with carbon nanotubes,” U.S. Patent Application Publication US 2009/0116171 A1, disclose electrolytic capacitors having electrodes made from free-standing CNT films.
While the potential applications are manifold for cohesive assemblies of carbon, and more particularly those of carbon nanotubes, several methods for producing such assemblies have also been investigated. However, these methods typically involve complex processes involving chemical modification of CNTs, reactions with hazardous gases at high temperatures, application of high pressures, use of highly corrosive materials, or other techniques that are either costly or difficult to scale up for manufacturing use.
For example, Smalley et al in “Method for producing self-assembled objects comprising single-wall carbon nanotubes and compositions thereof,” U.S. Pat. No. 7,048,999 B2, disclose CNT assemblies formed by a complex process of CNT end-cap removal and derivatization. Tohji et al in “Carbon nanotubes aggregate, method for forming same, and biocompatible material,” U.S. Patent Application Publication US 2007/0209093 A1, disclose a method for CNT aggregate formation involving exposure to fluorine gas followed by sintering at high temperature and pressure.
Liu et al in US 2009/0116171 A1, and Hata et al in “Aligned carbon nanotube bulk aggregates, process for production of the same and uses thereof,” U.S. Patent Application Publication US 2009/0272935 A1, disclose methods for preparing CNT assemblies that require the use of CNT forests grown by CVD processes on a substrate. These methods involve a sequence of solvent washing, pressing, and/or drying steps and are limited to the scale of the starting CNT forest. Furthermore, these assemblies are characterized by a predominant orientation or alignment of the CNTs, which imparts the assembly with anisotropic and largely unidirectional properties.
Whitby et al in “Geometric control and tuneable pore size distribution of buckypaper and bucky discs,” Carbon 46 (2008) pp. 949-956, disclose a frit compression method for forming CNT assemblies, which also requires high pressures. Also, the CNTs are not uniformly distributed within the assemblies, and the assemblies have large macropores and very high porosity (>80%).
Lastly, a method to form a solution of single-walled CNTs in sulfuric super-acids is disclosed by Davis et al in “Phase Behavior and Rheology of SWNTs in Superacids,” Macromolecules 37 (2004) pp. 154-160. A method is also disclosed to produce an entangled mat of CNT ropes by quenching in ether and filtering.
In summary, there exists a need for an improved method to prepare cohesive assemblies of carbon having good mechanical strength and integrity, and in particular carbon nanotubes, in a simple manner that allows the preparation of assemblies in desired shapes, and is scalable both in terms of the size of the individual assembly and for manufacturing quantities of assemblies. The present invention fulfills this need and provides further related advantages.